Wednesday, December 31, 2014

A lot of critical information here if you are a 5.56mm shooter like I am - everything you need to know regarding barrel length to twists ratio, bullet weight and propellant load . . . S.L.

As each set of measurements is recorded, the barrel was shortened (and re-crowned) an inch at a time and the pressure sensor moved to the resulting farthest location. In this photo, the barrel has been shortened to eight inches.

There has been a cultural shift from the 20-inch barrel length in the AR-15/M16 weapon systems chambered for the 5.56×45 NATO cartridge to progressively shorter barrels for the purpose of producing an increasingly more compact assault/entry weapon without resorting to a bull-pup design. Simple usage of these short-barreled weapons has shown the necessity for both sound and flash suppression, the intensity of which (in exceptionally short barrel lengths) approached the intensity of a flash-bang diversion device. This shift toward shorter barrels has resulted in the U.S. Army and Marine Corps adopting the 14.5-inch barreled M4 carbine with a re-design of the 5.56×45 from the 55 grain SS-109 to the 63 grain M855 ammunition to optimize this barrel length. The differing bullet design also necessitated a change in the rifling twist rate from the original 1:12 inches to 1:7 inches.

One of the authors routinely measures entrance chamber pressures in his company’s suppressors to calculate safety factors for different standard barrel lengths. A special mount holding the Kistler 6215 piezoelectric sensor clamps over the suppressor where a 2.5 mm hole has been drilled through the suppressor wall. Shown is a 5.56mm suppressor on a 10.5-inch barreled M16 in a Lead-Sled rest.